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AWS Basics Using CloudFormation (Part 3) – ELB and EC2

This is the third part of a 3-part article covering the basics of AWS through using CloudFormation. For the first part of this article, click here, and for the second, click here.

This is the third and final part in my AWS basics article. So far, I’ve covered CloudFormation and Amazon VPC. This time, I will cover Elastic Load Balancing (ELB), and Amazon EC2, the actual operational pieces that end up getting deployed and serve the web content. The final product is a basic virtual datacenter that load balances across two web servers, deployable with a single command through CloudFormation.

And once more, if you would like to follow along at home, remember to check out the template on the GitHub project.

Elastic Load Balancing (ELBs)

ELB has a feature set that is pretty much what could be expected from a traditional layer 7 load balancer, such as SSL offloading, health checks, sticky sessions, and what not. However, the real fun in using ELB is in what it does to make the job of infrastructure management easier.

As a completely integrated platform service, ELBs are automatically redundant, and can span multiple availability zones without much extra configuration. Metrics and logging are also built in, and can be sent to S3 or CloudWatch.

Other than that, there is not much to really hype up about ELB. Not to say that is a bad thing! So on with the CloudFormation entries.

ELBs in CloudFormation

After the gauntlet I ran with explaining the VPC entries in the sample CloudFormation stack, the ELB entry will be a breeze. Below is the ELB section.

The resource is of the AWS::ElasticLoadBalancing::LoadBalancer type. It is an internet-facing load balancer (as defined by Scheme), as opposed to an internal load balancer that would only be visible within the VPC. It’s also associated with the VCTSLabSubnet1 subnet, so that it can have public access, it does not affect the instances that it can connect to. The instances are defined in the Instances property, which contain references to the two named instances in the EC2 section of the template.

Health checking

The HealthCheck property marks an individual service as healthy (defined by HealthyThreshold) after 2 checks, which brings it back into the cluster; subsequently the health check will also mark a service as unhealthy after 2 failures (defined by UnhealthyThreshold). Note that although this is okay for the purpose that I am using it for, intermittent service failures may cause an undesirable flapping when thresholds are set this low. In that event, set HealthyThreshold to a value that ensures there have been enough successful checks to reasonably determine that the service is available.

Timeout controls how long to wait before marking an individual service as down if a response has not been received. Interval is the time to wait between checks. Both of these values are in seconds. In the example above, the health check waits 3 seconds before marking a service as failed, and the health check itself runs every 5 seconds.

The health check Target takes the syntax of SERVICE:PORT/urlpath. SERVICE can be one of TCP, SSL, HTTP, and HTTPS. /urlpath is only available for the last two (the first two being simple connect open checks and lacking any protocol awareness other than SSL). Also, the response to /urlpath needs to be a 200 OK response – anything else (even a 300 Redirect class code) is considered a failure. In the example above, a check against / over HTTP will be done on any EC2 instances to be sure that the service is up.

Listeners

The listener describes how clients connect to the load balancer and how those connects are routed to instances.

Here, connections come in to port 80 (defined by LoadBalancerPort) and are handled as HTTP connections (defined by Protocol). There are implications from this; namely the X-Forwarded-For HTTP header will be passed, and the connection is statefully passed across as a proxy. Use of HTTP on the front end also means that HTTP or HTTPS needs to be used on the back end. This is indeed the case; the listener is configured to send traffic to instances via HTTP on port 80 (defined by InstanceProtocol and InstancePort).

There are topics that are not covered in this article; namely having to do with SSL offloading (ie: using HTTPS as the front end or instance protocols), persistence, and back-end authentication. It would be wise to check out the Listeners for Your Load Balancer section of the ELB manual to get an idea of all available configurations for listeners.

EC2

There was a time, albeit a long time ago, that AWS was simply EC2 and not much else. Although, it should be noted that SQS was the first AWS service; Jeff Barr’s article on his first 12 years at Amazon is a good read on the launch dates of SQS, EC2, and S3.

Even in the face of today’s AWS massive platform service portfolio, I personally think it’s safe to say that EC2 still has a very major place at Amazon. It serves as the building block for services like ECS (AWS’s Docker service); the EC2 instances that make an ECS pool are, as of this writing, still visible to the end user and require some degree of management. Custom workloads may not fit the bill for use on zero-administration platforms like Lambda. Managed service providers that run their customers off AWS will have a need for the service for quite a long time to come.

EC2 is Amazon’s most basic building block, and the product that gave “Cloud Computing” its name (its acronym itself standing for Elastic Compute Cloud). It is a Xen-based virtualization platform, with features that in today’s world we now take for granted, such as host redundancy and per-use billing, to just name a couple. It set the standard for how a cloud platform handles instances – virtual machines are first rolled into base units called images (which under AWS is called an AMI, standing for Amazon Machine Image), from which instances are created with their own storage laid on top of it.

This small overview does not do the service justice, and there is no way that I would be able to cover all of EC2’s features in this document without losing sight of the goal of setting up a basic VPC with CloudFormation. I would recommend the EC2 documentation for coverage on these topics, in addition to watching this space, where I will more than likely cover these topics as need be.

EC2 in CloudFormation

And now, finally, I come to the last section in this part of the series – the EC2 section of the sample CloudFormation template.

Below is the definition of one of the two EC2 instances that are set up in the template, not counting the NAT instance.

EC2 instances are defined as the AWS::EC2::Instance instance type. The instance type is t2.micro, the smallest of the newer generation T2 instance types. Also, remember from the Mappings part of the CloudFormation section that the actual AMI to use is selected from the RegionMap map, based off the availability zone that this instance is launched in.

The KeyName is chosen from the supplied key name when the CloudFormation template was launched (it was either supplied on the command line or through the CloudFormation web interface).

The subnet (specified by SubnetId) is VCTSLabSubnet2, the private subnet, along with its SecurityGroupIds, which is in this case is the VCTSPrivateSecurityGroup private subnet security group (which is simply an allow all, as this group will have no internet access and will be interfacing with the NAT instance and the ELB).

Using userdata for post-creation work

The section after all the other aforementioned properties is where some of the real magic happens. The UserData property is used to create a post-installation shell script that updates the system (/usr/bin/yum update), installs apache (/usr/bin/yum -y install httpd), enables the service (/sbin/chkconfig httpd on), creates an index.html page with the server ID, and then finally injects a self-destructing cfn-signal command that gets run when the server reboots. This is a very simple way to get a fully deployed server in our example.

Note that there is a more complex configuration management system built right right into CloudFormation if using something more complex like Chef, Puppet, Ansible, Salt, or whatever is not possible. Check out AWS::CloudFormation::Init. Incidentially, this requires the cfn-init command be launched, which is not necessarily installed on all Linux AMIs (however is available usually thru pacakges, and is already on the system with Amazon Linux). Incidentally as well, cfn-init is generally launched through user data.

Finally, also note that user data needs to be base64 encoded – this is done by the Fn::Base64 section in the example.

Creation policies and dependencies

The last little bit that needs to be mentioned in regards to the EC2 instances are the creation policies and dependencies attached to them. These are not unique to EC2 instances (and hence, they are not properties of that specific resource type, as can be seen from their scope).

Consider the following scenario: the NAT instance has generally the same UserData as the EC2 instances – it updates and reboots as well. During the period that the NAT instance is rebooting, internet access will be unavailable to the 2 web instances in the private subnet. If all 3 instances were set to install at the same time without the web instances waiting for the NAT instance, it is plausible that there would be a time where the web instances would be attempting updates while the NAT instance was rebooting. This would, of course, break updates, and possibly the creation of the CloudFormation stack.

This is what creation policies and dependencies are for. Generally, when using user data, one does not want to count a resource as created until everything is done. In this case, that means the instance has had all of its software updated, any other software installed that it needs (ie: in the event of the web instances), and has been fully rebooted.

The CreationPolicy defined above waits until that is all done. Ultimately, by what it is defined there, it waits for one cfn-signal command to be run for the resource (defined by ResourceSignal), with a 10 minute Timeout (if the format looks weird, it’s because it is in ISO 8601 format). This gives the node enough time to fully update and restart.

And finally, the DependsOn attribute ties the web instances to the NAT instance. This will ensure CloudFormation waits until the NAT instance (referring to it by its named resource, VCTSLabNatGw) has completed creation and received its own cfn-signal before even attempting to create them, giving us an error-free template!

Conclusion

This concludes the intro article. I hope that you found the material informative!

Watch this space for much more in the way of coverage of AWS services as I continue my “world tour”. Not going to say 100% about what is next, but more than likely Route 53 will be on the radar shortly, as possibly will be an introduction to Identity and Access Management and Security Token Service, as both of the latter services are pretty important when organizing security on an AWS account these days, and there is a lot to digest.